1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device having conductors for analog signals and conductors for digital signals formed on a single circuit board. The present invention also relates to an audio appliance employing such a semiconductor integrated circuit device.
2. Description of the Prior Art
In a semiconductor integrated circuit device having conductors for analog signals and conductors for digital signals formed on a single circuit board, it is essential to take measures to reduce crosstalk between conductors in designing the layout of conductors. In conventional semiconductor integrated circuit devices, the common approaches to minimizing crosstalk are: (1) minimizing the area over which two conductors cross each other (see FIG. 3A), (2) laying a conductor to make a detour so as not to cross another conductor (see FIG. 3B), and (3) permitting only an analog signal conductor leading to a low-impedance circuit to cross a digital signal conductor (see FIG. 3C). It should be noted that, in FIGS. 3A to 3C, analog signal conductors are indicated by L1 and digital signal conductors are indicated by L2.
It is true that, with a semiconductor integrated circuit device in which one or more of the approaches mentioned above is adopted against crosstalk, it is possible to reduce transfer of noise from a digital signal conductor to an analog signal conductor and thereby achieve satisfactory transmission of an analog signal.
However, the approach (1) above has the disadvantage of increasing the resistance in a narrowed part of a conductor, resulting in an increased signal attenuation factor. The approach (2) above has the disadvantage of increasing the resistance in a detouring part of a conductor, resulting in an increased signal attenuation factor, and also has the disadvantage of unnecessarily enlarging the chip size. The approach (3) above has the disadvantage of permitting only a limited type of analog signal conductor to cross a digital signal conductor.
It is particularly difficult to adopt the approaches (1) to (3) mentioned above in an operational amplifier IC designed for use in an audio appliance ready for a multiple-channel sound system (such as a home-theater apparatus or television monitor apparatus ready for the 5.1-channel sound system). The reasons are, among others, that, in such circuitry, (i) the resistive components of analog signal conductors need to be minimized, (ii) detouring is not tolerated beyond a certain limit in designing the layout of multiple-channel analog signal conductors, and (iii) the input impedances of operational amplifier circuits to which analog signal conductors are connected are high. As a consequence, in the audio apparatus mentioned above, when a digital operation is performed (for example, to control the sound volume) during the playback of an audio signal (analog signal), noise may be transferred from a digital signal conductor to an analog signal conductor, producing audible noise in the audio output.
In addition to the already-mentioned approaches, there has conventionally been disclosed and proposed a technique for reducing crosstalk whereby a low-impedance shielding conductor is inserted between an analog signal conductor and a digital signal conductor (see, for example, Japanese Patent Application Laid-Open No. H5-47943).
It is true that, with the semiconductor integrated circuit device disclosed in the publication mentioned above, it is possible to reduce transfer of noise from a digital signal conductor to an analog signal conductor and thereby achieve satisfactory transmission of an analog signal.
However, in a common semiconductor integrated circuit device having conductors for analog signals and conductors for digital signals formed on a single circuit board, priority is almost always given to improving the transmission quality of digital signals and to improving the response, rather than to improving the transmission quality of analog signals. The semiconductor integrated circuit device disclosed in the publication mentioned above is no exception, because there, whereas digital signal conductors are formed of aluminum, which is a low-impedance material, analog signal conductors are formed of polysilicon, which is a high-impedance material. That is, in conventional semiconductor integrated circuit devices including the one disclosed in the publication mentioned above, analog signal conductors have never given priority over digital signal conductors in terms of the use of aluminum as their material. As a consequence, according to the prior-art technique mentioned above, analog signal conductors tend to have resistive components so high as to invite transfer of noise thereto. This, disadvantageously, diminishes the effect of the insertion of shielding conductors and also degrades the signal attenuation factor for analog signals.
Needless to say, in a case where digital signals having frequencies as high as 100 [MHz] or more are handled, there is no choice but to adopt the prior-art technique mentioned above to give priority to improving their transmission quality and to improving the response. However, in a semiconductor integrated circuit device such as an operational amplifier IC designed for use in an audio appliance or the like, where the highest priority is given to improving the transmission quality of analog signals (audio signals), on which the playback sound quality depends, and where the transmission quality of digital signals (for example, appliance operation signals having frequencies of 1 [MHz] or less) and related characteristics are of less importance, the technique against crosstalk disclosed in the publication mentioned above is not best-suited.
The applicant of the present invention once disclosed and proposed a technique of using, for signals having different frequencies, conductors laid in different layers (see Japanese Patent Application Laid-Open No. H11-238846). However, the technique disclosed in this publication is aimed, first and foremost, at giving redundancy to a low-frequency circuit in a digital circuit, and contributes only to a limited extent, with a well-devised circuit layout, to what the present invention is aimed at (i.e., reducing transfer of noise from a digital signal conductor to an analog signal conductor).